The present invention relates to fluoropolymer dispersions suitable for the cloths impregnation and the obtainment of cast films.
More particularly it relates to fluoropolymer dispersions having improved substratum wetting, i.e. with reduced presence of cissings, and minimum viscosity variation with temperature and critical thickness maintenance, necessary condition to obtain coatings of high thickness and free from cracks with few passages. Moreover the formulations obtainable from the fluoropolymer dispersions of the present invention are such to minimize the discoloration problems of the manufactured article.
It is known that the fluoropolymer dispersions and of PTFE in particular, are used to impregnate cloths and to coat supports able to thermally withstand to the sintering treatment that the fluoropolymer must undergo to form a continuous film. A particular case is formed by PTFE cast films, obtained by successive deposition of PTFE films brought to sintering and then removed from the support.
The use of fluoropolymer dispersions for the cloth coating (glass fiber, KEVLAR, etc.) and for obtaining films has been known for a long time.
One of the problems arising during the use of the fluoropolymer aqueous dispersions for the above purposes consists in assuring a very good and homogeneous covering of the support to be coated with the fluoropolymer. The greatest problems arise when a first layer of fluoropolymer has been deposited on the support and one has to continue to deposit fluoropolymer aqueous dispersion on the first fluorinated layer having low surface energy (for instance for the PTFE xcex3 is about 17 dyne/cm).
Another problem is to avoid that the fluoropolymer dispersion viscosity has a great dependence on the temperature in the range 10xc2x0-50xc2x0 C., preferably 20-35xc2x0 C. Indeed the temperature of the bath where the dispersion is placed can vary in this range during the deposition process, with negative consequences for the control of the impregnation process of the cloths or of the cast film formation. In general it is preferable that the viscosity absolute value of the formulated dispersion in this temperature range is not higher than 150-200 cPoise, preferably not higher than 100 cPoise, still more preferably lower than 60 cPoise. Industrial processes generally operate continuously and the drying and sintering conditions are fixed at the beginning. A variation of the dispersion viscosity during the time, involving a pick-up variation (amount of deposited dispersion) would oblige to a continuous adjustment of the initial drying and sintering conditions. This way to operate is not suitable from an industrial point of view. On the other hand if these continuous adjustements are not carried out, the quality of the final coating is very poor and besides it does not maintain constant during the time.
A further problem is to have a high critical thickness of the formulated dispersion to allow to deposit high dispersion thicknesses without the formation, during the sintering, of cracks and pinholes which lower the sealing and protection capability of the coating. A further charateristic which it is desirable not to be meaningfully modified is the colour of the final manufactured article, in particular of the cast films obtainable from the dispersions.
To solve these technical problems it is known in the art to use dispersions containing surfactants.
However the fluoropolymer dispersions usually commercialized, are not able of contemporaneously solving all the mentioned problems.
For instance polytetrafluoroethylene (PTFE) dispersions containing about 60% by weight of PTFE and containing as surfactant TRITON(copyright) X100, alkylphenoxyethoxylated surfactant commonly used, in an amount of 3% by weight, are not able to give coatings on cloths usable from the industrial point of view since the cloths contain cissings, i.e. zones not homogeneously coated.
Therefore the use of these cloths for instance in the foodsector is not possible since in these zones they do not show the desired antisticking characteristics.
Other used PTFE-based dispersions, which contain besides TRITON(copyright) X100 also other alkylphenoxyethoxylates, give a wetting increase but they introduce discoloration problems and viscosity increase.
The Applicant has unexpectedly and surprisingly found that it is possible to prepare fluoropolymer dispersions capable to overcome all the above technical problems.
It is an object of the present invention fluoropolymers aqueous dispersions comprising:
a) 1 to 10% of an hydrogenated nonionic surfactant;
b) 0.1 to 3% by weight of a fluorinated nonionic surfactant selected from the following classes:
Rfxe2x80x94Lxe2x80x94(OCH(R1)xe2x80x94CH(R2))nxe2x80x94Oxe2x80x94Axe2x80x83xe2x80x83(I)
xe2x80x83wherein:
n is an integer in the range 4-60, preferably 8-30;
L is selected from:
xe2x80x94(CF(Y)xe2x80x94COxe2x80x94O)pRxe2x80x2xe2x80x94; xe2x80x94(CF(Y)xe2x80x94COxe2x80x94NH)pRxe2x80x2xe2x80x94;
xe2x80x94(CF(Y)xe2x80x94SO2xe2x80x94NH)pRxe2x80x2xe2x80x94;
wherein: p is 0 or 1; Y can be F or CF3; Rxe2x80x2 is an alkylic radical C1-C5, linear or branched when possible;
R1, R2 can preferably be both H or the former H and the latter CH3,
Rf can be a radical of (per)fluoroalkane type from 4 to 20 C atoms or of (per)fluoropolyether type comprising repeating units randomly distributed along the polymer chain selected from:
xe2x80x94CF(X)CF2Oxe2x80x94 or xe2x80x94CFXOxe2x80x94, wherein X is equal to F or xe2x80x94CF3;
xe2x80x94CF2(CF2)zOxe2x80x94 wherein z is an integer equal to 2 or 3;
xe2x80x94CF2CF(ORfxe2x80x3)Oxe2x80x94 or xe2x80x94CF(ORfxe2x80x3)Oxe2x80x94 wherein Rfxe2x80x3 can be xe2x80x94CF3, xe2x80x94C2F5, or xe2x80x94C3F7;
A is selected from the following ones:
xe2x80x94H;xe2x80x94CH, xe2x80x94CH2OH; xe2x80x94CH2OCH2CH2OH; xe2x80x94CH2(OCH2CH2)nxe2x80x2xe2x80x3ORxe2x80x2 wherein nxe2x80x2xe2x80x3 is an integer in the range 2-15 and Rxe2x80x2 is H, CH3, COCH3; xe2x80x94CONHCH2CH2OH;
c) 25 to 75% by weight of water.
The Rf radical generally has a number average molecular weight from 250 to 1500, preferably from 400 to 1000.
The perfluoropolyether radicals Rf comprise a T end group selected from xe2x80x94CF3, xe2x80x94C2F5, xe2x80x94C3F7, ClCF2CF(CF3)xe2x80x94, CF3CFClCF2xe2x80x94, ClCF2CF2xe2x80x94 and ClCF2xe2x80x94.
In particular the following prfluoropolyether radicals Rf can be mentioned as preferred:
(a) Txe2x80x94O (CF2CF(CF3)O)a(CFXO)bxe2x80x94
wherein: X is F or CF3; a and b are integers such that the molecular weight is in the above range; a/b is in the range 10-100 when b is different from 0, and T is one of the above mentioned end groups;
(b) Txe2x80x94O(CF2CF2O)c(CF2O)d(CF2(CF2)zCF2O)hxe2x80x94
wherein: c, d and h are integers such that the molecular weight is in the above range; c/d is in the range 0.1-10; h/(c+d) is in the range 0-0.05, z is an integer equal to 2 or 3, and T is one of the above mentioned end groups;
(c) Txe2x80x94O(CF2CF(CF3)O)e(CF2CF2O)f(CFXO)gxe2x80x94
wherein: X is F or CF3; e, f, g are integers such that the molecular weight is in the above range; e/(f+g) is in the range 0.1-10, f/g is in the range 2-10, T is one of the above mentioned end groups;
(d) Txe2x80x94O(CF2O)j(CF2CF(ORfxe2x80x2)O)k(CF(ORfxe2x80x2)O)1xe2x80x94
wherein: Rfxe2x80x3 is xe2x80x94CF3, xe2x80x94C2F5, xe2x80x94C3F7; j,k,l are integers such that the molecular weight is in the above range; k+l and j+k+l are at least equal to 2, k/(j+l) is in the range 0.01-1000, l/j is in the range 0.01-100; T is one of the above mentioned end groups;
(e) Txe2x80x94Oxe2x80x94(CF2(CF2)zCF2O)sxe2x80x94
wherein s is an intger such as to give the molecular weight above indicated, z has the above meaning and T is one of the above mentioned end groups;
(f) Txe2x80x94O (CR4R5CF2CF2O)j, xe2x80x94
wherein R4 and R5 are equal to or different from each other and selected from H, Cl or perfluoroalkyl, for instance having 1-4 C atoms, jxe2x80x2 being an integer such that the molecular weight is that above indicated;
(g) Txe2x80x94O(CF(CF3)CF2O)jxe2x80x3xe2x80x94
jxe2x80x3 being an integer such as to give the moelcular weight above indicated.
These compounds and the methods for their preparation are described in GB 1,104,482, U.S. Pat Nos. 3,242,218, 3,665,041, 3,715,378 and 3,665,041, EP 148,482 and U.S. Pat. Nos. 4,523,039, 5,144,092.
The preferred nonionic surfactants of formula (I) are those in which L=CH2xe2x80x94CH2; n ranges from 8 to 12, R1=R2=H; Rf is a pefluoroalkyl from 5 to 10 carbon atoms, preferably from 6 to 8.
The preferred perfluoropolyether radicals of the present invention have the following structures:
Rfxe2x80x3xe2x80x94Oxe2x80x94(CF2CF(CF3)O)a(CF2O)b
ClC3F6O(CF2CF(CF3)O)a(CF2O)b
wherein a and b are such that the molecular weight ranges from 500 to 800, and Rfxe2x80x3 has the above meaning.
Preferably the amount of the component b) is in the range 0.15-2% by weight, the component c) is preferably in the range 35-55%.
The component a) can also be a mixture of two or more nonionic hydrogenated surfactants. For instance the following classes can be mentioned:
alkylphenoxypolyethoxylates having a different ratio between alkylic- and ethoxylic segment wherein the alkyl segment ranges from 6 to 15 carbon atoms and the ethoxylic segment ranges from 4 to 20 ethoxylic units; for instance octylphenoxyethoxylate is mentioned in which the ethoxy group generally consists of 10 units (commercial product TRITON(copyright) X100); or the nonylphenoxyethoxylate in which the ethoxy group generally shows nine units (commercial product RIOKLEN(copyright) NF8);
polyethoxylated aliphatic alcohols in which by aliphatic it is meant an alkylic chain from 5 to 20 carbon atoms and the ethoxylated segment generally has from 4 to 20 ethoxylic units (as reported in the Patent Application WO 96/30445),
alkyl substituted oxidized amines (see for instance U.S. Pat. No. 5,219,910).
The component b) can be used mixed with anionic fluorinated surfactants perfluorocarboxylic acid salts from 5 to 11 carbon atoms, for instance ammonium perfluorooctanoate; perfluorosulphonic acid salts from 5 to 11 carbon atoms; mono and bi carboxylic acid salts derived from perfluoropolyethers can be mentioned; the ratio by weight between nonionic and anionic surfactants ranges from 0.1 to 10, preferably from 0.5 to 2.
Among the fluoropolymers, polytetrafluorcethylene or tetrafluoroethylene copolymers can be mentioned. In particular, as TFE copolymers, the following ones can be mentioned:
A) modified polytetrafluoroethylene containing small amounts, generally in the range 0.01-3% by moles, preferably in the range 0.05-0.5% by moles, of one or more comonomers such as, for instance, perfluoropropene, perfluoroalkylperfluorovinylethers, vinylidene fluoride, hexafluoroisobutene, chlorotrifluoroethylene, perfluoroalkylethylenes;
B) tetrafluoroethylene (TFE) thermoplastic copolymers containing from 0.5 to 8% by moles of at least a perfluoroalkylvinylether, where the perfluoroalkylic radical has from 1 to 6 carbon atoms, such as for instance, TFE/perfluoropropylvinylether, TFE/perfluoromethylvinylether, TFE/perfluoroethylvinylether, TFE/perfluoroalkylethylene copolymers; alternatively to perfluoroalkylvinylether it can be used a fluorodioxole, preferably as defined hereinafter in D) 3); TFE amorphous copolymers with fluorodioxoles, preferably as defined below, the amount of fluorodioxole can range from 40 to 95% by moles;
C) tetrafluoroethylene thermoplastic copolymers containing from 2 to 20% by moles of a perfluoroolefin C3-C8, such as for instance the TFE/hexafluoropropene copolymer, to which small amounts (lower than 5% by moles) of other comonomers having a perfluorovinylether structure can be added (as described for instance in the U.S. Pat. No. 4,675,380);
D) tetrafluoroetilene thermoplastic copolymers containing from 0.5 to 13% by weight of perfluoromethylvinylether and from 0.05 to 5% by weight of one or more fluorinated monomers selected in the group formed by:
1)
RFOxe2x80x94CF=CF2xe2x80x83xe2x80x83(II)
xe2x80x83wherein RF can be:
i) a perfluoroalkylic radical containing from 2 to 12 carbon atoms;
ii)
xe2x80x94(CF2xe2x80x94CF(CF3)xe2x80x94O)r(CF2)rxe2x80x2xe2x80x94CF3xe2x80x83xe2x80x83(III)
xe2x80x83wherein r is in the range 1-4 and rxe2x80x2 is zero or in the range 1-3;
iii)
xe2x80x94Z(OCF(X))q(OCF2xe2x80x94CF(Y) )qxe2x80x2xe2x80x94Oxe2x80x94Txe2x80x83xe2x80x83(IV)
xe2x80x83wherein
the (OCFX) and (OCF2xe2x80x94CFY) units are randomly distributed along the chain; T, X and Y have the same meaning previously defined; Z represents xe2x80x94(CFX)xe2x80x94 or xe2x80x94(CF2xe2x80x94CFY)xe2x80x94;
q and qxe2x80x2, equal to or different from each other, are zero or integers in the range 1-10;
the average number molecular weight of the monomer is in the range 200-2,000.
iv)
xe2x80x94(CF2xe2x80x94CF(W)xe2x80x94O)txe2x80x94(CF2)2xe2x80x94(Oxe2x80x94CF(W)xe2x80x94CF2)txe2x80x2xe2x80x94Oxe2x80x94CF=CF2xe2x80x83xe2x80x83(V)
xe2x80x83wherein W is xe2x80x94Cl, xe2x80x94F or xe2x80x94CF3 and t and txe2x80x2 are zero or integers in the range 1-5; 
xe2x80x83wherein u is zero or an integer in the range 1-4;
2)
RFxe2x80x94CH=CH2xe2x80x83xe2x80x83(VII)
xe2x80x83wherein RF has the meaning described in 1);
3) a perfluporodioxole of formula: 
xe2x80x83wherein R3 is a perfluoroalkyl radical C1-C5; X1 and X2 are, independently from each other, a fluorine atom or xe2x80x94CF3, both X1 and X2 are preferably fluorine atoms.
The preferred weight ratio of the three classes of monomers forming the thermoplastic copolymers tetrafluoroethylene/perfluoromethylvinylether/fluorinated monomers (D) is the following:
perfluoromethylvinylether: 2-9%;
fluorinated monomers: 0.1-1.5%;
tetrafluoroethylene: complement to 100%.
Among the comonomers of formula (II), perfluoroethylvinylether, perfluoropropylvinylether and perfluorobutylvinylether can for instance be mentioned. The preferred comonomer of this class is the perfluoropropylvinylether.
The comonomers of formula (III) are described, for. instance, in the published European Patent Application No. 75,312. Examples of these comonomers are those in which r can be 1 or 2 and rxe2x80x2 is 2.
The comonomers of formula (IV) are obtained by dechlorination of the compounds of formula:
CF2Clxe2x80x94CFClxe2x80x94Oxe2x80x94Z(CFXO)qxe2x80x94(OCF2xe2x80x94CFY)qxe2x80x2xe2x80x94Oxe2x80x94T
which can be prepared as described in Example 11 of U.S. Pat. No. 4,906,770.
The comonomers of the class (V) can be prepared according to the procedures described in the British Patent 1,106,344. Among these comonomers the compound:
xe2x80x83CF2xe2x95x90CFxe2x80x94Oxe2x80x94(CF2xe2x80x94CF2)xe2x80x94Oxe2x80x94CFxe2x95x90CF2
can be mentioned.
The comonomers of the class (VI) can be prepared according to the procedures described in U.S. Pat. No. 4,013,689.
In the comonomers of formula (VII) RFxe2x80x94CHxe2x95x90CH2, the radical RF preferably contains from 2 to 6 carbon atoms. Examples of these comonomers are perfluorobutylethylene and perfluorohexylethylene.
The fluorinated comonomers 1-3 above described can be copolymerized with pure TFE and perfluoromethylvinylether to give terpolymers or in combination among each other to give tetrapolymers or copolymers having. a higher constituent complexity.
The tetrafluoroethylene copolymers can be prepared by radical polymerization in aqueous medium. It is particularly suitable to carry out the polymerization in aqueous phase according to the European Patent Application No. 247,379, or as described in the U.S. Pat No. 4,864,006.
The concentration of the obtained latex by polymerization is generally in the range 20-50% by weight and its viscosity is typically of few centipoises, such as for instance 5-10 centipoises.
The latex can be further concetrated by means of the prior art techniques, for instance by using conventional nonionic surfactants, such as for instance octylphenoxypolyethoxyethanol, kwnon on the market with the commercial name of Triton(copyright) X 100 commercialized by Union Carbide.
The results obtained with the present invention dispersions are unexpectd and surprising since:
not all the surfactants which lower the aqueous solution surface tension allow to reach the indicated property combination;
the noticed lowering, of the invention dispersion surface tension is minimum, it is not such as to be considered the wetting cause and it is not correlated to the application test success;
some known surfactants cause an undesirable variation of the dispersion viscosity or they must be added in high amounts, giving discoloration phenomena during the fluoropolymer sintering phase;
high critical thicknesses without the presence of cracks are obtained.